One product which acquires the application of multiple, parallel, uniform rows of adhesive is disposable diapers. In the manufacture of disposable diapers, parallel rows of pressure-sensitive adhesive are applied to a moisture impervious backing sheet of the diaper so as to adhere the backing sheet to the absorbent, non-woven pad of the diaper. To ensure secure attachment of these layers by means of an economical quantity of adhesive, while obtaining an acceptable visual appearance of the resulting product, parallel adhesive rows in the form of beads or a spray pattern must be accurately positioned along the backing sheet and formed in uniform width and thickness.
It has been the practice in prior art methods of making disposable diapers to employ a metering gear head positioned above a moving layer of the plastic backing sheet to apply multiple, parallel beads of pressure-sensitive adhesive to the plastic backing sheet for subsequent attachment to an absorbent pad. Metering gear heads include a plurality of spaced discharge orifices which are each supplied with adhesive from a separate gear pump for applying multiple, parallel beads of adhesive on the plastic backing sheet. Although metering gear heads apply adhesive beads on a substrate with good accuracy, and dispense beads of uniform size and width, there are several problems in the use of metering gear heads for the manufacture of disposable diapers.
One problem with metering gear heads is that they are relatively heavy and bulky, making it difficult to mount them in close proximity on a diaper manufacturing line. The size of metering gear heads is attributable, in part, to the fact that each bead they dispense on a surface requires a separate gear pump and an associated drive motor to control the flow of adhesive forming the bead. The use of separate gear pumps for dispensing each bead contributes to high cost of the metering equipment, and results in a relatively complex metering device. As a result, the cost for maintaining the equipment is very appreciable.
Another problem with metering gear heads is that their operation results in a substantial waste of adhesive in the manufacture of disposable diapers. Cut-outs are made at intervals in the backing sheet and non-woven layer of the diaper material for the leg holes, and a substantial savings of adhesive could be realized if the adhesive was applied intermittently to the outer edges of the diaper to leave a gap without adhesive where the leg holes are cut.
If metering gear heads are operated intermittently to dispense adhesive, they produce a substantial cut-off drool when turned off, and then do not immediately provide consistent flow when turned back on. A disposable diaper manufactured by an intermittently operated metering gear head would therefore have uneven and/or varying width adhesive beads on both sides of the backing sheet where the leg hole is cut, which is visually unacceptable. As a result, metering gear heads are operated continuously in the manufacture of disposable diapers and apply continuous multiple beads across the entire width of the backing sheet of the diaper.